氮素添加对贝加尔针茅草原土壤团聚体微生物群落的影响

大气氮沉降增加作为全球气候变化的重要因素,其对土壤生态系统影响的研究受到了生态学家的广泛关注。土壤微生物是有机物分解和养分循环的主要参与者,在维持土壤的功能多样性和可持续发展方面发挥着重要的作用。氮沉降的激增会引起土壤微生物群落结构和功能的改变。土壤中营养物质在不同团聚体组分中分布的不均匀,为微生物提供了空间异质微生境。为揭示草原土壤不同粒径团聚体中微生物群落分布及其对氮素添加响应特征。自2010年起,在内蒙古贝加尔针茅草原典型地段设置N₀（0 kg hm^-2 a^-1）、N₁₅（15 kg hm^-2 a^-1）、N₃₀（30 kg hm^-2 a^-1）、N₅₀（50 kg hm^-2 a^-1）、N₁₀₀（100 kg hm^-2 a^-1）、N₁₅₀（150 kg hm^-2 a^-1）6个氮素添加处理模拟氮沉降野外控制试验。采用磷脂脂肪酸（phospholipid fatty acid,PLFA）法测定>2 mm、0.25-2 mm和<0.25 mm 3个粒径土壤团聚体中微生物PLFA含量,探讨氮素添加对土壤团聚体微生物群落结构的影响。结果表明：氮素添加提高了土壤碳、氮含量,降低了土壤pH。氮素添加显著提高了0.25-2 mm土壤团聚体微生物群落磷脂脂肪酸总量、真菌磷脂脂肪酸含量和真菌/细菌（Fungi/bacteria,F/B）、革兰氏阳性菌/革兰氏阴性菌（Gram-positive bacteria/gram-negative bacteria,G⁺/G^-）的比值（P<0.05）,降低了土壤团聚体微生物Margalef丰富度指数（P<0.05）。相关性分析表明,土壤团聚体微生物总PLFAs、真菌PLFAs含量、G⁺/G^-、F/B与土壤有机碳、全氮含量呈显著正相关关系,与C/N值负相关。综合研究表明,连续8年氮素添加显著提高了土壤有机碳和全氮含量、降低了土壤pH;提高了0.25-2 mm土壤团聚体真菌群落,土壤有机碳、全氮的固持与真菌群落的增加有关。     

氮添加对马尾松人工林土壤团聚体氮矿化及土壤酶活性的影响

土壤氮库是生态系统氮素重要的源和汇。以三峡库区马尾松（Pinus massoniana）人工林为研究对象,从团聚体视角出发分析土壤养分和酶活性对氮添加的响应规律,以及相应的变化对氮矿化的影响,为预测该地区在大气氮沉降持续增加的背景下土壤氮动态提供参考。设置4种量的氮添加处理（N₀：0 kg N hm^-2 a^-1;N₃₀：30 kg N hm^-2 a^-1;N₆₀：60 kg N hm^-2 a^-1;N₉₀：90 kg N hm^-2 a^-1）,将土壤按粒径分为>2000 μm （大团聚体）、250-2000 μm （小团聚体）和<250 μm （微团聚体）3个组分的团聚体,观察团聚体氮矿化特征。结果表明：（1）与对照相比,N₃₀和N₆₀处理提高了有机质（SOM）含量,但土壤SOM和全氮（TN）含量在N₉₀下开始出现下降;氮添加降低了土壤速效磷（aP）含量,在小团聚体中表现最为显著。除微团聚体中的POD和NAG以外,其余3种酶的活性均在N₃₀和N₆₀处理之下被提高。（2）土壤平均净硝化速率整体高于土壤平均净氨化速率;大团聚体和小团聚体中净氨化速率在氮添加处理后显著降低,大团聚体净硝化速率低于其他两个粒径;土壤净氮转化速率在N₉₀处理下最高。（3）土壤养分和无机氮含量与土壤酸性磷酸酶（AP）、N-乙酰-β-D-葡糖苷酶（NAG）、过氧化物酶（POD）、硝酸还原酶（NR）和脲酶（UE）的活性呈显著相关,酶活性变化是多因子综合作用的结果;RDA分析显示,UE与土壤净氨化速率存在显著正相关,NAG和POD是与净氮转化速率分别存在显著正相关和显著负相关的关键土壤酶。综上所述,硝化作用是土壤净氮转化的主要贡献者,微团聚体在土壤氮矿化中发挥主要作用,NAG和POD是改变土壤净氮转化的主要生物酶。此外,氮添加会引起土壤氮素的流失,引起土壤的磷限制,并对土壤养分循环产生显著影响。     

氮添加对巴音布鲁克高寒湿地土壤微生物量和酶活性的影响

随着全球氮沉降速率的快速增加,已对陆地生态系统微生物群落活性和代谢产生了深刻的影响。因此迫切需要了解全球气候变化敏感区土壤中微生物量和酶活性对氮添加的响应。为此,以中亚干旱区巴音布鲁克高寒湿地为研究对象,在保护良好的高寒湿地选择沼泽(S)、沼泽草甸(SM)和草甸(M)3种湿地类型布设野外原位氮添加试验(施氮浓度分别为0、8、16 kg N hm^-2 a^-1),探究短期氮添加对土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳/氮(MBC/MBN)、微生物商(QMB)、土壤蛋白酶、脲酶、碱性磷酸酶、H₂O₂酶和蔗糖酶活性的影响。结果表明:(1)高寒湿地不同湿地类型土壤微生物量和酶活性存在显著差异,其中SM土壤MBC、MBN、MBC\\N、QMB较S和M区高,对酶活性而言,SM和M区土壤蛋白酶和碱性磷酸酶活性较高,M区H₂O₂酶和脲酶活性较高。(2)氮添加显著增加了3种湿地类型中土壤MBC和MBN,其中MBC增加了7.00%-119.00%,MBN增加了8.03%-38.26%。氮添加仅显著增加了S和SM区土壤MBC/N和QMB (增加了24.68%-113.10%),但抑制了M区土壤MBC/N和QMB (抑制了8.93%-10.36%)。(3)氮添加显著增加了3种湿地类型土壤中脲酶、蛋白酶和H₂O₂酶活性,分别增加了7.25%-59.63%、4.71%-58.55%和34.70%-157.27%。但是氮添加对土壤碱性磷酸酶活性无显著影响。对蔗糖酶而言,N1处理增加了S区土壤蔗糖酶活性(增加了58.58%),而N2处理显著降低了22.72%。氮添加对SM和M区蔗糖酶活性无显著影响。(4)结构方程模型的结果显示,氮添加直接增加了土壤微生物量和酶活性。而随着湿地类型的变化(S-SM-M)直接和间接(通过pH)增加了酶活性;湿地类型的变化还通过影响pH、有机碳和有效养分间接增加了土壤微生物量。总之,氮添加和湿地类型可直接或间接的影响着土壤微生物量和酶活性。其中,土壤pH和有机碳是微生物量和酶活性变化的主要影响因素。本研究可为中亚干旱区高寒湿地应对未来气候变化的措施的制定提供技术参考。     

连续氮添加14年对温带典型草原土壤碳氮组分及物理结构的影响

全球氮沉降对生态系统造成了深远的影响,研究长时间氮沉降对草地生态系统土壤理化特征的影响有助于加强生态系统对氮沉降响应的长效机制的理解。通过连续14年长期施加N0（0 g N m^-2 a^-1）、N2（2 g N m^-2 a^-1）、N4（4 g N m^-2 a^-1）、N8（8 g N m^-2 a^-1）、N16（16 g N m^-2 a^-1）、N32（32 g N m^-2 a^-1）六种浓度尿素模拟氮沉降,并将土壤分成0-10、10-20和20-40 cm三个深度土层,研究温带草原生态系统土壤碳氮组分及物理结构对氮添加的响应及其相互关系,结果表明：（1）氮添加显著降低0-10 cm土壤酸碱度及土壤微生物量碳含量,N32相比N0分别下降了27.63%和58.40%（P<0.05）;各土层总有机碳和全氮含量对氮添加处理无显著响应,0-10 cm土层显著高于20-40 cm土层。（2）同一土层深度不同梯度氮添加处理显著增加土壤无机氮离子含量（P<0.05）,0-10 cm土层铵态氮含量N32相比N0增加了88.72%,20-40 cm土层硝态氮含量N32相比N0增加了19.55倍,土壤深度与氮添加对无机氮离子含量影响具有显著的交互效应。（3）同一土壤深度不同梯度氮添加处理土壤粒度分形维数及土壤团聚体差异不显著,相关分析表明土壤碳氮元素含量与土壤结构显著相关。土壤碳氮组分在适宜浓度氮添加的增加趋势说明氮添加在一定程度上可能促进土壤理化性质的改良,氮添加对土壤物理结构的影响还需要进一步的深入研究。     

川西亚高山不同林龄粗枝云杉人工林土壤微生物生物量及酶活性

林分结构随林龄增加发生变化,致使土壤微环境变化,进而对土壤微生物生物量和酶活性产生直接或间接影响。以川西亚高山米亚罗林区25、40、50、60年生粗枝云杉（Picea asperata）人工林表层（0-20 cm）土壤为研究对象,分析不同林龄表层土壤微生物生物量碳（MBC）、氮（MBN）和酶活性的变化以及驱动土壤酶活性变化的主要环境因子,为人工林生态系统恢复效果评价及经营管理提供科学依据。结果表明：随着林龄的增加,MBC和MBN先上升后下降,在40年生达到最大;β-葡萄糖苷酶（βG）、β-N-乙酰氨基葡萄糖苷酶（NAG）和多酚氧化酶（PHO）活性与微生物生物量碳氮变化趋势相似,其中βG、NAG活性在50年生人工林达到最大,PHO活性于40年生人工林达到最大;50年生人工林过氧化物酶（PEO）活性显著低于其他林龄人工林;纤维素水解酶（CBH）活性随林龄增加而显著增加。冗余分析显示,碱解氮（AN）、pH和可溶性有机碳（DOC）是影响土壤酶活性变化的主导因子,分别解释了土壤酶活性变异的65.4%、9.7%和7.6%。综上可知,林龄对粗枝云杉人工林表层土壤微生物生物量和酶活性变化产生了显著影响,60年粗枝云杉林地力呈现衰退态势。为此,可对近60年的粗枝云杉人工林增加碳氮元素（尤其是氮素）的输入,以提升土壤质量和酶活性。     

氮沉降对杉木人工林土壤可溶性有机质数量和结构的影响

氮沉降作为现在乃至未来气候变化的趋势之一,其可能深刻影响土壤可溶性有机质的数量和结构。选取我国中亚热带杉木人工林不同深度土壤（0-10 cm和10-20 cm）进行氮沉降试验,利用光谱技术研究氮沉降对土壤可溶性有机质数量和结构的影响。试验设对照（CT,0 kg hm^-2 a^-1）、高氮（HN：80 kg hm^-2 a^-1）、低氮（LN：40 kg hm^-2 a^-1）3种处理。结果表明：（1）在0-10 cm和10-20 cm土层,HN、LN处理的土壤可溶性有机碳和可溶性有机氮含量显著高于CT。（2）在0-10 cm和10-20 cm土层,1月时HN、LN处理的芳香性指数和腐殖化程度都显著高于CT,而4月时HN、LN处理的芳香性指数和腐殖化程度都显著低于CT。除了氮含量的直接影响外,RDA （冗余分析）表明,两土层中土壤含水量、pH和土壤有机碳是驱动氮沉降对土壤DOM数量和结构的重要环境因子。因此,氮沉降对土壤DOM的影响是复杂的,未来尤其应该注重氮沉降对生态系统影响的季节模式。     

模拟氮沉降对土壤酶活性和微生物组成的影响

氮沉降改变了草地生态系统的氮(N)素循环过程,由此带来的生态学效应已成为当前研究的热点。以乌鲁木齐周边短期围封草地为研究对象,通过模拟氮沉降实验,分析了自由放牧地和围封草地土壤酶活性和微生物组成,结合土壤养分及化学计量特征,探讨了氮沉降对短期围封草地土壤微生物组成及酶活性的影响,为该地区放牧草地的保护、恢复及管理提供理论依据。结果表明:(1)土壤有机碳(SOC)、全氮(TN)、全磷(TP)含量随围封年限的增加总体呈升高趋势,表明围封有利于提高土壤养分含量。与中国草地平均值相比,该草地土壤碳氮比(C/N)相对较高,碳磷比(C/P)、氮磷比(N/P)相对较低,表明该草地土壤有机质分解良好,有利于土壤碳(C)、磷(P)的释放,而土壤N素较为缺乏。(2)就不同围封年限而言,围封3年草地5-20cm层土壤真菌数量高于其它样地;围封3年草地表层土壤蔗糖酶与过氧化氢酶活性最高;围封7年草地放线菌数量最多,说明围封能够促进土壤微生物生长及酶活性的提高。(3)氮素添加对土壤真菌具有抑制作用,N5(4.6gN m^-2 a^-1)、N10(9.8gN m^-2 a^-1)处理显著增加了各样地土壤细菌数量,氮素添加对围封7年草地0-10cm层土壤放线菌无显著影响,而氮沉降显著增加了其它样地5-20cm层土壤放线菌数量,其中N5、N10处理下促进作用最明显;氮素添加对该草地土壤脲酶、蔗糖酶、过氧化氢酶均具有促进作用,N5、N10处理促进作用最明显。综合分析表明,氮沉降可直接或间接影响土壤微生物及酶活性,短期围封作为一种草地管理手段,对退化草地生态系统的修复具有一定作用,并可通过改善土壤理化性质、调节养分含量及其化学计量比来加速退化草地的恢复。     

有机物添加下太岳山油松林土壤微生物元素组成的稳态分析

在山西太岳山地区,向油松林土壤中分别添加生物炭、玉米秸秆、蒙古栎叶、油松叶、木屑等5种有机物,测定各处理的土壤养分、酶及微生物生物量等指标,研究外源有机物添加下土壤酶化学计量特征及微生物元素组成的内稳性。结果表明: 添加木屑显著增加了土壤N(17.1%)、P(37.6%)含量,显著增加了微生物生物量碳(118.0%)、氮(41.0%)、磷(176.6%)。C、N、P获取酶(β-1,4-葡萄糖苷酶、β-1,4-N-乙酰氨基葡萄糖苷酶、亮氨酸氨基肽酶、酸性磷酸酶)活性总体上随添加有机物C/N值(生物炭<蒙古栎叶<油松叶<玉米秸秆<木屑)的增加而增加,其化学计量变化受土壤养分状态及微生物生物量的调控。酶活性相对比例及矢量特性表明,研究区微生物生长受到P的限制,且添加有机物没有缓解P的制约作用。微生物生物量碳、氮及化学计量比C∶N、C:P、N∶P属于绝对稳态型,而微生物生物量磷处于非稳态。微生物通过改变酶的分配策略保持微生物体元素及比例的相对稳定,仅有微生物生物量磷对土壤养分变化表现出不稳定性,可能因为P是研究区微生物生长的限制性元素。     

湿地土壤微生物功能多样性及碳氮组分对长期氮输入的响应

氮输入对湿地生态系统碳氮循环具有重要影响,研究湿地土壤微生物功能多样性及碳氮组分对氮输入的响应,对于明确湿地土壤碳氮循环微生物驱动机制具有重要意义。依托长期野外氮输入模拟试验,利用Biolog-ECO微平板技术,分析不同浓度氮输入:N1(6 g N m^-2 a^-1)、N2(12 g N m^-2 a^-1)和N3(24 g N m^-2 a^-1)对湿地土壤表层(0-15 cm)和亚表层(15-30 cm)微生物碳源代谢活性、功能多样性和碳氮组分的影响。结果表明:N2处理显著提高了亚表层土壤微生物碳源代谢活性和McIntosh指数,N3处理显著降低了表层土壤微生物Shannon指数和Shannon-evenness指数。随氮输入浓度增加湿地表层土壤微生物对糖类的利用率显著降低,N3处理表层土壤微生物对胺类的利用率以及亚表层土壤微生物对醇类的利用率显著提高。N1处理显著提高了湿地表层土壤全氮和微生物量碳含量;N2、N3处理显著提高了土壤铵态氮、硝态氮含量;N3处理显著降低了土壤pH值。湿地土壤pH、总碳、溶解性有机碳含量是影响微生物碳源代谢活性和功能多样性的重要因素,土壤溶解性有机碳、铵态氮、全氮含量、含水率是影响微生物碳源利用变化的主要因子。     

氮添加对樟子松人工林氮转化及相关功能基因丰度的影响

土壤氮(N)的有效性是影响土壤微生物群落结构以及土壤氮循环的重要因子。为探索N添加对樟子松人工林氮素转化及N功能基因(NFGs)表达的影响及其作用机制,该文以塞罕坝千层板林场的樟子松人工林为研究对象,进行了2年的氮添加处理,设置4个不同氮添加水平0、1、5、10 g N·m^-2·a^-1,分别记作N0、N1、N5、N10,采用功能基因微阵GeoChip 5.0系统及室内土壤培养法,探讨了土壤NFGs对氮添加的反应及其对氮转化过程的影响。结果表明:(1)与N0相比,中低N添加处理(N1、N5)促进了氨化(ureC、nirA、nrfA)、硝化(amoA)和反硝化(norB)相关基因的相对丰度,高N处理(N10)则抑制了所有NFGs的表达。(2)相关分析表明,N1、N5的促进作用与土壤有机碳(SOC)、硝态氮(NO₃^--N)和微生物生物量碳(MBC)显著相关,N10处理显著降低了所有氮转化过程NFGs的相对丰度,这种负面影响与溶解性有机碳(DOC)、MBC含量的减少有关。(3)与氮转化基因丰度规律趋势相似,N1和N5处理显著增加了净N硝化、净N矿化以及N₂O的排放速率,但N10促进作用不明显,表明氮添加对氮转化的促进作用存在阈值。(4)多元回归分析进一步表明,amoA-AOB和MBC是影响净N硝化的关键因素,ureC、nirK和MBC是影响净氮矿化的关键因素,narG、nirS是影响N₂O排放的关键因素。综上,N添加可提高促进樟子松人工林的氮转化及提高部分特定酶功能基因的相对丰度,但氮添加水平存在阈值,当施用10 g N·m^-2·a^-1时,氮转化受到抑制,添加5 g N·m^-2·a^-1是促进樟子松人工林土壤N转化的较佳水平。     

Vermicomposting in the management of pig-waste in Hong Kong

The treatment and disposal of pig-waste in Hong Kong has received much attention in recent years but, following any of the presently used treatment processes, solids remain to be further stabilized. Vermicomposting is a waste stabilization technique which converts waste into potentially recyclable materials such as worm protein and worm casts. The earthworm, Pheretima asiatica, can stabilize most of the solids arising from the treatment of pig-waste, including raw pig manure, suggesting that vermicomposting has a high potential as a unit process in the management of pig-waste in Hong Kong.S.H. Wong is with the Environmental Protection Department, Hong Kong; and D.A. Griffiths is with the Department of Botany, University of Hong Kong, Hong Kong.     

Removal of wastewater pathogen indicators in a constructed wetland in Leon, Spain

This paper studies the seasonal presence and removal of the pathogenous micro-organisms Escherichia coli, total coliforms (TC), Clostridium perfringens (Cp), faecal streptococci (FS), Giardia cysts, Cryptosporidium oocysts and helminth eggs, in a constructed wetland treatment system. The removal efficiency of this system with respect to the indicator micro-organisms achieved maximum values in spring and autumn at 99.9% for E. coli and TC, respectively, in winter at 97.0% for FS, in summer at 100% for Clostridium and throughout the year, also at 100%, in the case of Giardia cysts, Cryptosporidium oocysts and helminth eggs. In general, very low protozoan and helminth egg counts were found, and the system demonstrated efficient reduction of the wastewater indicator pathogens.     

Inbreeding in a french peasant population

Genealogical pedigrees were collected from 976 farming couples who lived in a canton of central France in the XIXth and XXth century. Several methods (genealogical, isonymic and probability of origin of genes) were used to appraise the evolution of the biological structure of this population, which has gone through two phases. The first phase from 1800 to the World War I, shows gene flows between the farming population of the canton and those of neighbouring rural districts which are linked to an increase in consanguineous marriages. The second phase, from 1920 onwards, shows important gene flow from distant areas. This trend implies a decrease in consanguineous marriages.     

Lactic acid bacteria in fermented foods in Thailand

Traditional fermented foods (fish, meat and vegetable products), produced by many different processes, are eaten in many parts of Thailand. Lactic acid bacteria are responsible for the souring and ripening of these foods. Homofermentative strains of Lactobacillus pentosus, L. plantarum and Pediococcus pentosaceus are dominant in foods with low salt concentrations whereas P. halophilus strains are present in foods containing high salt. Strains of Lactobacillus sake, other Lactobacillus spp., P. acidilactici and P. urinaeequi are frequently found. Heterofermentative strains of L. brevis, L. confusus, L. fermentum, L. vaccinostercus, other Lactobacillus spp., and of Leuconostoc spp. are distributed as minor bacteria and strains of Staphylococcus, Enterococcus and Halobacterium are occasionally isolated.S. Tanasupawat is with the Department of Microbiology, Faculty of Pharmaceutical Sciences. Chulalongkorn University, Bangkok 10330, Thailand; K. Komagata is with the Department of Agricultural Chemistry, Tokyo University of Agriculture, Sakuragaoka 1-1-1, Setagaya-ku, Tokyo 156, Japan.     

New species and new combinations in Peruvian Orchidaceae

seventeen new species and combinations are proposed in the generaChondrorhyncha, Cischweinfia, Cochlioda, Eloyella, Encyclia, Kefersteinia, Koellensteinia, Macroclinium, Rodriguezia, Solenidiopsis, andStenia. All new species are illustrated. A key is provided for 2-flowered species ofMacroclinium, PeruvianSigmatostalix, and PeruvianStenia. Solenidium (Solenidiopsis) peruvianum Schltr. is lectotypified.     

Phylogenetic relationships in theCampanulales based onrbcL sequences

Phylogenetic relationships within the angiosperm orderCampanulales were investigated by comparative sequencing of the chloroplast generbcL. CompleterbcL sequences were obtained for ten species in six families within the order. These data were analyzed along with previously publishedrbcL sequences from other taxa (for a total of 117 species) within the subclassAsteridae and outgroups, producing 32 equally parsimonious trees. A subset consisting of 44 of these taxa was then chosen and more rigorous analyses performed, resulting in four equally parsimonious trees. Results indicate that two major clades roughly corresponding to traditionally circumscribedAsterales andCampanulales exist as sister taxa. In particular, therbcL trees indicate thatSphenoclea is not a member ofCampanulales orAsterales, thatPentaphragma is more closely allied toAsterales thanCampanulales, that theCyphiaceae are not monophyletic, thatCampanulaceae andLobeliaceae are not sister taxa, and thatStylidiaceae are correctly placed withinCampanulales.     

Nectary structure and nectar characteristics in someBignoniaceae

The nectary structure and chemical nectar composition of 15 species belonging to 12 genera ofBignoniaceae are analyzed. All taxa bear a conspicuous nuptial nectary surrounding the ovary base. The secretory tissue is mostly supplied by phloem branches. The stomata are located in the middle and upper part of the nectary epidermis with an homogeneous distribution. The nuptial nectary is proportionally large in relation to the ovary (15–30%), disregarding the nectary volume. Most species have extranuptial nectaries in both inner and outer surfaces of the calyx. Both kinds of nectaries lack a vascular tissue that straightly supplies them. Nuptial nectar concentration (wt/wt) ranges from 19 to 68%. Sugars and amino acids are found in all species. Half of the species have hexose predominant nectars, the remaining sucrose predominant. Phenols are detected in only three species, whereas reducing acids exclusively inTecoma stans. Alkaloids and lipids were never detected. Extranuptial nectar chemical composition is analyzed in two species:Dolichandra cynanchoides andPodranea ricasoliana. Bees constitute the main flower visitors of the species studied whereas hummingbirds were seen visiting three species. A correlation analysis is performed with the data obtained. There are a few significant correlations which indicate a parallel increase of three parameters: the longer the flower length, the more voluminous the nectary and the higher stomata number, independently of the floral biotype. Phenograms are obtained using 24 floral characters including nectary and nectar data. The clusters obtained do not reflect taxonomic relationships but are useful in the understanding of animal-plant interactions when the flower biotype is considered.This paper is based on a chapter of a doctoral thesis presented at the University of Córdoba (Argentina).     

Synthetic oligonucleotide probes for detection of mercury-resistance genes in environmental freshwater microbial communities in response to pollutants

Mercury-resistance genes were detected byin situ hybridization using new synthetic oligonucleotide probes specific formerA andmerB genes according to the published sequences of the corresponding enzymes. These DNA probes were used for the detection of specific mercury-resistant microorganisms isolated from the Rhine River which had been polluted 3 years previously in 1986. Mercuric reductase and organomercurial lyase genes persist in the bacterial genome even after the disappearance of the pollutant but are absent in axenic amoebae. A total of 49 bacterial isolates showed DNA homologies with the³²P-labelled DNA probes and 15 free-living amoebae were selected due to their harboured symbiotic mercury-resistant bacteria.     

Mycoflora of Argentinian corn harvested in the main production area in 1990

Corn (Zea mays) is the main cereal produced in and exported from Argentina. The risk of contamination by mycotoxins is related to the mycoflora associated with the corn kernels. This paper reports on the identification of internal and external mycoflora of corn kernels harvested in the main production area in Argentina in 1990. A mycological survey was carried out on 178 corn samples, from five locations in that area and the isolation frequency and relative density of the prevalent fungal genera compared. GenusFusarium was the most prevalent component of the internal seedborne mycoflora in the five locations.Penicillium was prevalent in all locations, taking into account the frequency. However, this genus was predominant only in two locations, when the relative density was considered. The predominantFusarium wasF. moniliforme and the most frequently isolated species ofAlternaria, Aspergillus andPenicillium wereA. alternata, A. flavus andP. decumbens, respectively.Diplodia species were not isolated from any of the samples.